Electronic circuits containing variable impedance elements are used in many applications. These variable impedance elements are usually in the form of variable resistors, also called potentiometers. Circuits using variable inductors or capacitors are also widely used. These variable impedance elements are usually manually adjusted to provide a selected impedance so as to affect some aspect of the circuit in which they are located. For example, a potentiometer may be set to a value which maximizes a signal generated at a node in a given circuit.
Manual adjustment of potentiometers is usually unsatisfactory in circuits under the control of data processing systems or other external electric circuits where ongoing adjustment of the potentiometer is necessary for circuit operation. The data processing system often must change the value of the variable impedance element in a time that is short relative to the time required to complete a manual adjustment of the variable impedance element. Manual adjustment also requires the presence of an operator. Operators are prone to error. Furthermore, in many situations manual adjustment is impractical. Remote control of resistance by a computer or digital system is needed in many applications.
A potentiometer can be controlled by mechanically adjusting motors or other actuators. Although these potentiometers relieve the need for an operator, they are still unsatisfactory in many applications. The time to make an adjustment is still too long for many applications. In addition, the long term reliability of such electromechanical devices is not sufficient for many applications requiring variable impedance elements. In addition, such systems are often too costly and consume too much real estate for many applications.
Digital potentiometers have been developed as a solution to the above problems. These digital potentiometers generally comprise a network of resistors that are selectively connected to a wiper terminal by a network of transistors, all of which are integrated onto a single chip of a semiconductor. Because fixed-values resistors are used and because the wiper position is selected by one or more transistors, the resistance value between a wiper and a main terminal of a digital potentiometer can only have a finite number of values. As an example, a 16-value digital potentiometer may comprise 15 equal-value resistors connected in series to form a series resistor stack, with the stack being connected between the two main terminals of the potentiometer. A select transistor is then coupled between each internal node of the series-resistor stack and the wiper terminal, and between each main terminal and the wiper terminal, for a total of 16 select transistors. One of the select transistors is set in a conducting state to select one point along the series-resistor stack. As can be seen by this example, the number of resistors and transistors required to implement a digital or solid-state potentiometer increases linearly with the desired number of discrete values. In general, the semiconductor chip area and cost of implementing a digital potentiometer increase, and the number of resistors and transistors increase, as the number of discrete values increases.
Digital potentiometers also have limited accuracy, due to limitations associated with microelectric fabrication technology. For example, some digital potentiometers specify as much as 20% lot to lot variation in resistance values. The step size between steps also varies to a great extent. This results in inaccuracy in the resistance actually achieved.